This invention relates to electrochemical cells and especially to high-energy, liquid cathode, non-aqueous lithium electrochemical cells free from highly toxic materials.
Since the 1950's a great deal of research has been done to raise the specific energy of batteries. Higher energy chemicals were employed for the electrode materials and of course much more stable electrolytes had to be found to withstand these strongly oxidizing and reducing chemicals. The relatively easily decomposed water of aqueous batteries was replaced by inert, aprotic solvents both organic and inorganic. Suitable electrolytes were identified combining sufficient conductance and electrochemical stability. Because of its low equivalent weight, strong reducing power, relatively high melting point and good rate capability, lithium emerged as the favored anode material and was coupled with a variety of solid and liquid oxidants. In the 1970's the Li/SO.sub.2 and Li/SOCl.sub.2 liquid-cathode batteries were confidently promoted as combining both high specific energy and power. Compared with the aqueous batteries, almost an order of magnitude increase in specific energy was achieved together with superior storability, constancy of voltage, and low temperature operation.
However, as industry delivered large numbers of these SO.sub.2 and SOCl.sub.2 batteries to U.S. Government agencies for test and use, some serious safety problems became evident. During use or storage the batteries would occasionally ignite or explode venting toxic gases. Consequently, their use became restricted and in some cases prohibited. The chemical events triggering the observed hazards are quite complex and still puzzle researchers. Intensive study has focused on clarifying these hazard mechanisms to allow the safe harnessing of high energy lithium batteries. Researchers are also exploring alternate cathode materials that are completely new and potentially superior to SO.sub.2 and SOCl.sub.2 in terms of both safety and efficiency.
One such effort is the work of Peled (U.S. Pat. No. 4,224,389). He circumvents the corrosiveness and toxicity of SO.sub.2 and SOCl.sub.2 by using aprotic, reducible, organic compounds free from these drawbacks. The cathode materials listed in his Examples comprise four haloethanes and propylene glycol-1,2-sulfite with butyrolactone and tetrahydrofuran as cosolvents and lithium perchlorate, tetrafluoroborate and chloride as conducting salts. A serious inadequacy of Peled's cathode materials however is their low, delivered coulombic capacity. For his six Examples of the above five depolarizers, cathode capacities ranged from 0.02 to 0.2 Ah per ml of carbon current collector at 2.3 V versus a Li anode. This compares unfavorably with the 0.8 to 1.2 Ah per ml at 3.0 V routinely obtained from Li/SOCl.sub.2 cathodes. Another attempt in the same direction was made by Doddapaneni (U.S. Pat. No. 4,439,503). He employed N,N-dichloroethyl carbamate as cathode depolarizer, no cosolvent and tetraalkylammonium perchlorate and lithium hexafluoroarsenate as conducting salts. His patent emphasized the sulfur-free nature of this depolarizer and its likely freedom from the thermal runaway hazards that have been associated with elemental sulfur produced in discharged SO.sub.2 and SOCl.sub.2 batteries. It would be desirable to produce safe, stable, sulfur-free lithium batteries having greater cathode capacities.